Chemical mechanical polishing slurry for polishing copper layer on a wafer

ABSTRACT

A chemical mechanical polishing slurry is provided for the copper layer on a wafer. The slurry contains colloidal silica and a chemical etching agent composed of hydrogen peroxide, acetic acid, and phthalic acid. The hydrogen peroxide oxides the surface of the copper layer. The acetic acid then reacts with the copper oxide to form copper acetate. This selective and functional chemical reaction mechanism can speed up the polishing removal rate and reduce scratches. The phthalic acid functions as both a pH buffering agent and a complexing agent to make the reaction concentration at each point of the wafer surface more homogeneous. Therefore, the copper layer during the chemical mechanical polishing process has a high removal rate and uniformity.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a chemical mechanical polishing (CMP) slurryand, in particular, to a slurry with a special chemical etching agentand colloidal silica for the CMP process of the copper layer on a wafer.

2. Related Art

As IC devices become smaller in size, the metal wires on chips are alsominiaturized. However, smaller wire widths result in larger resistanceand narrower separations result in larger capacitance. As the sizes ofdevices continue to shrink, there will be more signal delays. Therefore,copper wires with better conductivity have replaced conventionalaluminum wires in order to increase the signal transmission speed.

For a high-density multiple-layer copper wire production process, onecan employ chemical mechanical polishing (CMP) to satisfy therequirement of a uniform wafer. The basic principle is to use a chemicaletching agent in the slurry to react with the wafer surface. Along withthe abrasive mechanical actions of polishing particles, the dielectriclayers, barriers, or metal layers on the wafer are removed in order toachieve global planarity of the wafer. In the abrasive mechanism, thechemical and mechanical forces have to form an optimized dynamicalinteraction in order to reach high-quality polishing results. Simplyusing chemical reactions cannot achieve the global planarity. Likewise,considering only the mechanical actions will reduce the removal rate andare likely to scratch the wafer surface. To have appropriate materialremoval energy, it is desirable to have a slurry that can reduce thestructural strength on the surface of the material through chemicalreactions. This can reduce the stress (including normal forces and shearstress) required to take away polishing particles in the materialpolishing and removal process. It can further achieve the goals of noscratch, high removal rate, and global planarity.

The CMP slurry used in wafers is comprised of a slurry (chemical etchingagent) and polishing particles. The slurry is responsible for chemicaletching, while the polishing particles are responsible for mechanicalpolishing. Tests of the polishing slurry include the polishing removalrate, uniformity, surface scratches, and stability. Moreover, the slurryhas to be cheap and safe for industrial uses.

As a polishing slurry for the copper layer, most of the polishingparticles in the prior art are fumed alumina which have a high rigidityand irregular shapes (as in the U.S. Pat. Nos. 6,217,416 and 6,432,828)or fumed silica (as in the U.S. Pat. No. 6,309,560). This type ofpolishing particles synthesized by burning is expensive. Since they arehard to become dispersed, one usually has to add a separating agent.Although they have a higher removal rate in polishing, scratches areoften left on the wafer surface.

The polishing removal rate of colloidal silica is not as good as fumedalumina or fumed silica; nonetheless, the particles have a uniform size,a better separating property, and a cheaper price. They have a sphericalshape and a lower rigidity. Therefore, scratches can be avoided. Theonly problem in the prior art is that no appropriate chemical etchingagent has been found for the colloidal silica.

SUMMARY OF THE INVENTION

In view of the foregoing, an objective of the invention is to provide alow-cost and highly efficient polishing slurry for the chemicalmechanical polishing (CMP) of the copper layer on a wafer.

To achieve the above objective, the disclosed CMP slurry comprisescolloidal silica and a chemical etching agent composed of hydrogenperoxide, acetic acid, and phthalic acid. The chemical mechanicalremoval mechanism is to have the hydrogen peroxide and the copper layersurface react to form copper oxide. The acetic acid then reacts with thecopper oxide to form copper acetate to be readily removed (the aceticacid does not react with copper). Afterwards, the colloidal silicapolishing particles remove the copper acetate from the wafer surface.The phthalic acid functions as both a pH buffering agent and acomplexing agent of the polishing slurry, making the reactionconcentration at each point of the wafer surface more homogeneous.Moreover, the required concentration of the chemical etching agentaccording to the invention is very low. The cost can thus be greatlyreduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more fully understood from the detaileddescription given hereinbelow illustration only, and thus are notlimitative of the present invention, and wherein:

-   -   Table 1 shows the solid content and pH value of the colloidal        silica in various particle diameters;    -   Table 2 shows the effects of polishing particles of various        abrasive diameters on the removal rate and the non-uniformity;    -   Table 3 shows the effects of different solid contents of the        colloidal silica on the removal rate and the non-uniformity;    -   Table 4 shows the effects of slurries with the same composition        but different pH values on the removal rate and the        non-uniformity; and    -   Table 5 shows the effects of the composition and concentration        of the chemical etching agent in the slurry on the removal rate        and the non-uniformity.

DETAILED DESCRIPTION OF THE INVENTION

According to a preferred embodiment of the invention, the chemicalmechanical polishing (CMP) slurry contains:

-   -   (1) nano-scale colloidal silica with a concentration of 0.5˜10        wt %, the average particle diameter being 10˜100 nm;    -   (2) hydrogen peroxide with a concentration of 0.6˜2.5 V %;    -   (3) acetic acid with a concentration of 0.1˜1 V %; and    -   (4) phthalic acid with a concentration of 0.1˜0.8 wt %.

Moreover, the concentrations of hydrogen peroxide, acetic acid, andphthalic acid satisfy (hydrogen peroxide)/(acetic acid)=0.6˜25 and(acetic acid)/(phthalic acid)=0.125˜10. The pH value of the polishingslurry is 2.5˜5.5. The above configuration can achieve better polishingefficiency.

The polishing machine used in subsequent analyses is the CMP-300Pmachine made by Chung-Sun Institute of Science & Technology. We also usean 8-inch floating polishing head. Other details are as follows. Thedown force is 3 psi. The platen speed is 47 rpm. The head speed is 49rpm. The slurry supply rate is 150 ml/min. The polishing pad is RodelIC-1400. The polishing moisture time is 30 sec. The removal rate anduniformity are measured using NAPSON RT-80/RG-120 surface resistancemeter.

Test 1

The current test evaluates the effects of colloidal silica particlesizes on polishing results. The solid content of colloidal silica invarious sizes and their pH values are indicated in Table 1. Theconcentrations of the hydrogen peroxide, acetic acid, and phthalic acidare fixed to 1.2 V %, 0.375 V %, and 0.2 wt %, respectively. Theconcentration of the polishing particles is 3.0 wt %. The pH values andpolishing results of the slurries are given in Table 2. The experimentsshow that the disclosed slurry has a high removal rate and achieves ahigh uniformity. The removal rate is maximized when the particlediameter is 35.7 nm; however, its uniformity is not as good as thosewith an abrasive diameter of 19.7 nm and 90.5 nm. TABLE 1 Abrasivediameter (nm) Solid content (wt %) pH 19.7 14.78 11.59 35.7 22.87 3.7169.8 30.4 11.16 90.5 28.22 11.09

TABLE 2 Cu removal rate Abrasive diameter (nm) pH (Å/min) Non-uniformity(%) 19.7 4.80 4196 4.05 35.7 2.90 6383 7.81 69.8 3.25 5457 7.95 90.53.35 4991 5.93Test 2

This test evaluates the effects of the solid content of the colloidalsilica on the polishing removal rate and the uniformity. The abrasiveparticle diameter of all slurries is fixed to 90.5 nm. Theconcentrations of the hydrogen peroxide, acetic acid, and phthalic acidare fixed to 1.2 V %, 0.375 V %, and 0.2 wt %, respectively. The pHvalue and polishing results are shown in Table 3. It can be seen thatthe removal rate does not increase significantly when the abrasive slidcontent is 1˜3 wt %. Both the removal rate and the non-uniformityincrease simultaneously when the solid content is greater than 3 wt %.TABLE 3 Abrasive solid Cu removal rate content (wt %) PH (Å/min)Non-uniformity (%) 1 3.04 4900 2.78 3 3.35 4991 5.93 6 3.35 6334 6.57Test 3

In this test, we want to see the effects by changing the pH value of theslurry in the same composition. The polishing particles in all slurrieshave a concentration of 3 wt % and an abrasive diameter of 90.5 nm. Theconcentrations of the hydrogen peroxide, acetic acid, and phthalic acidare fixed to 1.2 V %, 0.375 V %, and 0.2 wt %, respectively. Theslurries are then added with diluted ammonia to adjust their pH values,as listed in Table 4. The results show that the removal rate decreasesas the pH value increases. The non-uniformity, on the other hand,increases with the pH value. TABLE 4 pH Cu removal rate (Å/min)Non-uniformity (%) 3.35 4991 5.93 4.00 4693 7.91 5.00 3974 7.83Test 4

This test checks the effects of changing the composition andconcentration of the chemical etching agent. The polishing particles inall slurries have a concentration of 3 wt % and an abrasive diameter of90.5 nm. The pH values and polishing results of all prepared slurriesare shown in Table 5. It shows that the uniformity is not good when onlyacetic acid is added. The removal rate is greatly enhanced while thenon-uniformity is reduced for slurries that have both acetic acid andphthalic acid. When the concentration ratios (hydrogen peroxide)/(aceticacid) and (acetic acid)/(phthalic acid) are fixed, simultaneouslyincreasing the concentrations of the hydrogen peroxide, acetic acid,phthalic acid can increase the removal rate and the uniformity. Withboth the concentration ratio (hydrogen peroxide)/(acetic acid) and thephthalic acid concentration being fixed, increasing the concentrationratio (acetic acid)/(phthalic acid) can increase the removal ratewithout affecting the uniformity to a good approximation. With the sameconcentrations of hydrogen peroxide and acetic acid, decreasing theconcentration ratio (acetic acid)/(phthalic acid) (or increasing thephthalic acid concentration at the same time) can increase the removalrate and the uniformity. TABLE 5 (hydrogen (acetic hydrogen aceticperoxide)/ phthalic acid)/ Cu removal Non- peroxide acid (acetic acid(phthalic rate uniformity (V %) (V %) acid) (wt %) acid) pH (Å/min) (%)0.8 0.25 3.2 0 — 4.01 2475 11.44 1.2 0.375 3.2 0 — 3.82 3476 11.66 1.60.5 3.2 0 — 3.69 4207 8.73 0.8 0.35 2.28 0 — 3.87 3444 10.83 0.8 0.451.78 0 — 3.76 4261 8.42 0.8 0.25 3.2 0.2 1.25 3.20 3825 4.12 1.2 0.3753.2 0.2 1.875 3.35 4991 5.93 1.6 0.5 3.2 0.2 2.5 3.24 6011 5.29 1.20.375 3.2 0.3 1.25 2.98 5661 3.92 1.6 0.5 3.2 0.4 1.25 3.07 6522 3.70

From the above tests, one can readily see that the disclosed slurrycomposition can achieve the goals of high removal rates and uniformity.

It should be emphasized that the CMP slurry for the copper layer on awafer first have the hydrogen peroxide interact with the copper surfaceto form copper oxide. The acetic acid then reacts with the copper oxideto form copper acetate, which is then removed by colloidal silicapolishing particles. This can achieve a high removal rate. The inventionfurther proposes to use the phthalic acid as the pH buffering agent andcomplexing agent in the slurry to increase the polishing uniformity atthe same time.

Although the disclosed embodiment uses colloidal silica as the polishingparticles in the slurry, other appropriate polishing particles can beemployed in the chemical etching agent to enhance the removal rate andthe uniformity.

Certain variations would be apparent to those skilled in the art, whichvariations are considered within the spirit and scope of the claimedinvention.

1. A chemical mechanical polishing (CMP) slurry for polishing the copperlayer on a wafer, comprising polishing particles, hydrogen peroxide,acetic acid, and phthalic acid; wherein the polishing particles are of aparticle diameter of 10˜100 nm and include a concentration of 0.5˜10 wt% colloidal silica; wherein the concentration of the hydrogen peroxideis 0.6˜2.5 V %, and wherein the pH value is 2.5-5.5. 2-4. (canceled) 5.The CMP slurry of claim 1, wherein the concentration of the acetic acidis 0.1˜1.0 V %.
 6. The CMP slurry of claim 1, wherein the concentrationof the phthalic acid is 0.1˜0.8 wt %.
 7. (canceled)
 8. The CMP slurry ofclaim 1, wherein the concentration ratio of hydrogen peroxide to aceticacid is 0.6˜25.
 9. The CMP slurry of claim 1, wherein the concentrationratio of acetic acid to phthalic acid is 0.125˜10.
 10. The CMP slurry ofclaim 5, wherein the concentration of the phthalic acid is 0.1˜0.8 wt %.11. The CMP slurry of claim 5, wherein the concentration ratio ofhydrogen peroxide to acetic acid is 0.6˜25.
 12. The CMP slurry of claim6, wherein the concentration ratio of hydrogen peroxide to acetic acidis 0.6˜25.
 13. The CMP slurry of claim 10, wherein the concentrationratio of hydrogen peroxide to acetic acid is 0.6˜25.
 14. The CMP slurryof claim 5, wherein the concentration ratio of acetic acid to phthalicacid is 0.125˜10.
 15. The CMP slurry of claim 6, wherein theconcentration ratio of acetic acid to phthalic acid is 0.125˜10.
 16. TheCMP slurry of claim 10, wherein the concentration ratio of acetic acidto phthalic acid is 0.125˜10.
 17. The CMP slurry of claim 11, whereinthe concentration ratio of acetic acid to phthalic acid is 0.125˜10. 18.The CMP slurry of claim 12, wherein the concentration ratio of aceticacid to phthalic acid is 0.125˜10.
 19. The CMP slurry of claim 13,wherein the concentration ratio of acetic acid to phthalic acid is0.125˜10.